The goal of this proposal is to determine the molecular basis for innate immunity in the insect model organism, Drosophila melanogaster. Upon infection, insects mount a self-defense response by mobilizing blood cells and producing antimicrobial peptides to neutralize the invading microorganisms. This insect self-defense response is similar to mammalian innate immune response. The innate immune system recognizes common features of microorganisms such as lipopolysaccharide and peptidoglycan and stimulates the production of protective molecules to fight the infection. In mammals, the innate immune system is also essential for the mobilization of B and T lymphocytes of the acquired immune system. A prominent example that demonstrates the conservation between insect and mammalian innate immunity is Toll and Toll-like receptor signaling. Toll was first identified as an essential component in Drosophila embryonic development and the same Toll signaling was later shown to regulate antimicrobial response in larvae and adults. Eleven mammalian Toll-like receptors have been identified and they are essential for mammals to recognize various microbial compounds. While many components of the insect Toll and mammalian Toll-like receptor signaling pathways have been identified, critical aspects of Toll and Toll-like receptor signaling are still unknown. We will use the highly manipulate Drosophila as a model system to elucidate the mechanism of Toll signaling during infection. The three specific aims of this proposal are: (1) Test whether the Spatzle-Toll interaction is essential for transmitting the signal of infection; (2) Determine whether multimerization is the activation mechanism for the receptor Toll; and (3) Examine whether heterodimers of Drosophila NF-?B-related proteins mediate Toll and Imd signaling. The successful achievement of the proposed research will provide important insights into the conserved immune processes and the design of agonists and antagonists to modulate the response to pathogens in humans. [unreadable] [unreadable] [unreadable]